1. Technical Field
The present invention relates to a wiring substrate manufacturing method.
2. Description of Related Art
In recent years, manufacturers have eagerly produced a semiconductor package utilizing a wiring substrate (also referred to herein as a “wiring substrate assembly”), in which a conductor layer and a resin insulating layer are laminated alternately in at least one layer (i.e., a laminated layer combining the alternately laminated conductor layer and resin insulating layer) respectively on at least one principal surface of a core layer, and then a solder resist layer is formed on the outermost surface thereof, i.e., a so-called resin-made wiring substrate. Then, a semiconductor device is mounted thereon.
The semiconductor device is connected electrically to the wiring substrate via respective solder bumps that are formed on pads in a semiconductor device mounting portion on a principal surface of the wiring substrate. In contrast, external terminals connected electrically to a base substrate or inserted in sockets and connected electrically thereto are formed on a back surface side of the wiring substrate. Here, according to a package mode of the external terminals, the wiring substrates are classified into a ball grid array (BGA), a pin grid array (PGA), etc.
A wiring substrate of the PGA type can be obtained by inserting pins into respective opening portions that are formed in a solder resist layer of the wiring substrate assembly, and then electrically connecting the pins to an outermost conductor layer exposed from the opening portions respectively.
A wiring substrate of the BGA type can be obtained by applying a fluxing process to portions of the outermost conductor layer exposed from opening portions formed in the solder resist layer of the wiring substrate assembly, if necessary, to remove the oxide film, then mounting solder balls on the respective exposed portions of the conductor layer, and then applying a reflow soldering process to the solder balls.
However, in the case of the wiring substrate of the BGA type, when the solder balls are formed directly on the portions of the conductor layer, adhesion of the solder balls to the portions of the conductor layer cannot be improved by the reflow soldering process, and thus the solder balls get out of (i.e., are dislodged from) their original mounting positions. For example, in some cases the electrical and mechanical connection between the wiring substrate and the base substrate cannot be sufficiently maintained.
In order to deal with such problem, a method has been proposed in which the fluxing process is applied to the portions of the conductor layer exposed from opening portions formed in the solder resist layer on the wiring substrate assembly, if necessary, then a predetermined solder paste is printed on the portions of the conductor layer. Then, respective portions of an underlying layer located under the solder balls are formed by reflowing the solder paste. Then, the solder balls are mounted on the respective portions of the underlying layer, and then the solder balls are connected to them by applying the reflow soldering process (see: JP-A-2006-173143 Official Gazette). In this case, the solder balls are connected to the respective portions of the conductor layer via the portions of the underlying layer, and therefore adhesion of the solder balls to the portions of the conductor layer can be improved rather than the case where the solder balls are connected directly to the portions of the conductor layer mutually.
However, the solder paste is formed like a hemispherical shape by the reflow soldering process. Therefore, a contact area between the underlying layer formed of such solder paste and the solder ball mounted thereon is decreased, and adhesion between them cannot be sufficiently maintained or improved. As a result, like the above, in some cases, the solder balls get out of (i.e., are dislodged from) their original mounting positions, and thus the electrical and mechanical connection between the wiring substrate and the base substrate cannot be sufficiently maintained.
A degree at which the solder ball is exposed from the opening portion is increased as a depth of the opening portion is decreased and/or a thickness of the underlying layer, particularly a thickness in the center portion, is increased. Thus, a shearing force that acts to the solder ball in the lateral direction is increased. Therefore, there is such a tendency that, as a depth of the opening portion is decreased and/or a thickness of the underlying layer is increased, the above phenomenon becomes more conspicuous.
In this case, when the underlying layer formed of a hemispherical solder paste is pressed by a predetermined planarizing machine, such underlying layer can be made flat to some extent, nevertheless the complicated and extra process, i.e., the use of the planarizing machine, is needed. Thus, there is such a problem that the total processes of the manufacturing method become complicated. Even when the underlying layer is made flat, a thickness of the entire underlying layer is increased, particularly, when a depth of the opening portion is small, adhesion between the solder ball and the underlying layer is not enough, and thus the solder balls get out of (i.e., are dislodged from) their original mounting positions. As a result, the problem such that the electrical and mechanical connection between the wiring substrate and the base substrate cannot be sufficiently maintained and cannot be sufficiently improved.